With accelerated business practices and the globalization of the marketplace, there is an ever increasing need for around the clock business communications and operations. As such, corporate data repositories must be able to provide critical business data at any moment in time in the face of interruptions caused by hardware failure, software failure, geographical disaster, or the like. To achieve the necessary data continuity and resilience for the present global marketplace, businesses utilize remote data repositories to backup and store critical business data.
One conventional method of data backup and storage is magnetic tape backup. At a business center, an amount of data, such as a day or a week, is transferred to a magnetic tape medium that is then stored remotely offsite. However, the magnetic tape medium is cumbersome to fetch in the event of a disaster and often requires significant amount of business center down time to restore the lost data.
Another conventional method utilized to avoid down time and provide disaster protection is database replication. With database replication, the database management system can make informed decisions on whether to write data to multiple local storage devices or a local storage device and to a remote storage device, but such synchronization comes at a significant performance penalty. The technique of writing data to multiple storage devices simultaneously is known as mirroring. In this manner, critical data can be accessible at all times. Ensuring transaction and record consistency often results in data transmission latency when a large number of data transmissions to remote sites are necessary with each database update. Consequently, application performance is slowed to unacceptable levels. In addition, database replication only performs replication only on data in the database and not on data in user files and system files. A separate remote copy facility is utilized to replicate such user files or system files.
In yet another data replication technique known as redundant array of independent disks (RAID), a host, such as a server or workstation, writes data to two duplicate storage devices simultaneously. In this manner, if one of the storage devices fails, the host can instantly switch to the other storage device without any loss of data or service. Nevertheless, to write to two duplicate storage devices simultaneously when one storage device is local and the other is remote is burdensome.
The mirroring of data to a distant location often faces remote data transmission limitations. For example, data transmission using a small computer system interface (SCSI) is limited to twenty-five meters. Typically, a SCSI parallel interface is used for attaching peripheral devices, such as a printer or an external storage device to a computer. Thus by utilizing a computer's SCSI port a computer can perform data mirroring by simultaneously writing to an internal storage device and an external storage device. Although discrete SCSI extenders are available, they become cumbersome and expensive beyond one or two remote connections.
One data transmission connection that is used for remote mirroring of data with an external storage device is the enterprise systems connection (ESCON) for use in mainframe systems. Unfortunately, ESCON has a maximum range of sixty kilometers.
A further example of a remote data transmission connection that is used for distant mirroring of data is fiber arbitrated loop (FCAL), which can distribute loop connections over 100 kilometers when properly equipped. Nevertheless, these data transmission connections do not provide the necessary long distance separation between an operational work center and the data repository to overcome regional disasters such as earthquakes, tornadoes, floods, and the like.
The above shortcomings can be overcome by use of a dedicated transmission medium between two sites, such as a high-speed fiber optic cable. However, most high speed transmission mediums are dedicated to telecommunications traffic. Moreover, the cost for a dedicated high-speed link makes such a choice prohibitive.
Another obstacle associated with long distance data mirroring is latency. That is, the round trip delay required to write data to the distant location and to wait for the remote storage device to be updated before mirroring the next data block. Typically, the latency is proportional to the distance between the two sites and can be heightened by intermediate extenders and communication protocol overhead. Consequently, application response slows to an unacceptable level.
A further obstacle to long distance data mirroring is compatibility among remote storage devices involved in the mirroring. As a result, a host having a data replication facility may replicate a data structure to one volume of the remote storage device at a time. Moreover, the host may not replicate the data further than the first remote storage device due to compatibility issues surrounding data transmission rates or host platform compatibility. These burdens place significant limitations on data protection schemes that require multiple remote storage devices.